1. Field of the Invention
The present invention relates to oral compositions such as topical solutions or pastes for application in the dental office, cavity sealers, gels for ultrasonic treatment, mouthwashes, prophylactic pastes, dentifrices, dental flosses and desensitizers of teeth. More particularly it relates to oral compositions containing stannous salts and water-soluble fluoride salts, and having the effect of inhibition of dental caries upon application.
2. Description of the Prior Art
Fluoride compounds have found widespread usage as effective ingredients for inhibiting dental caries. The addition of those fluoride compounds to drinking water, toothpastes, topical solutions and the like has been successful in the reduction of dental caries. Among those fluoride compounds, fluoride salts which contain stannous ions (e.g. stannous fluoride) have been reported to cause an increase in the fluoride uptake by the dental enamel and consequently in acid-resistance of the enamel after treatment as compared with fluoride salts which do not contain stannous ions (e.g. sodium fluoride). This fact suggests that fluoride salts with stannous ions are effective for reducing dental caries. Recently, it has also been reported that a highly insoluble film or deposits of Sn.sub.3 F.sub.3 PO.sub.4 are formed on the tooth surface, such film or deposits coming from the reaction of stannous fluoride with HPO.sub.4.sup.2- ion which has been formed from dissolution of part of the enamel by bringing the tooth surface into contact with an aqueous solution containing stannous fluoride for an extended period of time.
However, the fluoride compounds providing stannous ions such as stannous fluoride are chemically unstable. In the situation of an aqueous solution containing stannous fluoride as an example, its stability during storage and under heat deteriorates rapidly, followed by ready occurrence of insoluble, whity precipitants which are amorphous substances such as stannous oxide, stannous hydroxide and the like. This is less reactive with the dental enamel. Due to the gradual decrease in the effectiveness during storage, stannous fluoride becomes unsuitable as a dental caries inhibitor.
It has been the conventional practice to maintain stannous ions in a stable, available condition for easy reaction with the enamel. One means lies in the addition of any acid or acids in a relatively large quantity such that an oral composition may have a pH of less than 2. Low pH (below 2) in the composition, however, creates an obstacle to oral acceptability since it is excessively acidic. The other means is to mask the stannous ions with the aid of any chelating agent or agents such as EDTA, pyrophosphates and tripolyphosphates, and to thereby minimize hydrolysis of the stannous compounds. Such chelating agent or agents are liable to provide very stable complexes with the stannous ions, resulting in decreased availability of free or active stannous ions which are effective for inhibiting dental caries.
More advanced means have been proposed to surmount the above noted difficulties. U.S. Pat. No. 3,105,798 discloses a dentifrice composition consisting essentially of a water-soluble fluoride salt, stannous tin and a water-soluble source of six carbon aldonate groups capable of forming water-soluble complexes with stannous tin, the molar ratio of the aldonate group to stannous tin being in the range of from about one:one to about three:one, the molar ratio of stannous tin to fluoride ions being greater than one:one, said dentifrice having a pH of from about 5 to 7. Crystals of sodium pentafluorostannite (NaSn.sub.2 F.sub.5) obtained by reacting one mole of sodium fluoride with two moles of stannous fluoride are described in U.S. Pat. No. 3,490,866. These known oral compositions have not been widely recognized in effectiveness in the reduction of dental caries. The use of stannous salts of polyphosphonic acids such as methanediphosphonic acid or ethane-1-hydroxy-1, 1-diphosphonic acid described in U.S. Pat. No. 3,549,677 is also deficient in that production of the stannous salts of polyphosphonic acids is complicated and expensive.
The prior art compositions containing stannous ions and fluoride ions are effective to some extent for dental caries reduction, but their effectiveness is not so extremely high. Moreover, they require repeated application because of their lower reactivity to the tooth surface upon application or readily decreased retention of effectiveness.
Accordingly, an oral composition should be expected which possesses excellent effects on the inhibition of dental caries and which retains its effectiveness for an extended length of time without repeated application.